In fiber optic systems and other optical systems, optical measurement devices (which can also be referred to as spectrum analyzers) are used for various applications. Exemplary optical measurement devices include Optical Spectrum Analyzers (OSAs), Optical Channel Monitors (OCMs), Optical Performance Monitors (OPMs), etc. Generally, the functionality of optical measurement devices is to measure and assess the quality of various signals at different wavelengths (or frequencies) over a given optical spectrum. In Wavelength Division Multiplexing (WDM) systems, optical measurement devices are used to assess the given channels over the optical spectrum at various transmission points. WDM and Dense WDM optical systems typically use an optical spectrum of about 1530 nm and 1565 nm, which corresponds to the Erbium Doped Fiber Amplifier (EDFA) bandwidth. Conventionally, fixed channels are established on a wavelength grid in the optical spectrum, such as defined in ITU Recommendation G.694.1 “Spectral grids for WDM applications: DWDM frequency grid” (02/12), the contents of which are incorporated by reference herein. Also, new flexible grid (“flex-grid”) approaches are also now incorporated in ITU G.694.1.
The flexible grid allows a mixed bit rate or mixed modulation format transmission system to allocate frequency slots with different widths so that they can be optimized for the bandwidth requirements of the particular bit rate and modulation scheme of the individual channels. Such flexible grid deployments contemplate Nyquist or super-Nyquist spacing between channels on the optical spectrum. If each channel occupies BW amount of optical spectrum and the center frequency between adjacent channels is Δf, the super-Nyquist spacing is when Δf is less than BW and Nyquist spacing is when Δf is equal to BW. Practically, Nyquist or super-Nyquist spacing leads to close spacing or even slight overlap between adjacent channels, leading to optical measurement devices being unable to distinguish between adjacent channels.
Optical measurement devices are susceptible to aging effects such as where the frequency measurement on the received optical spectrum shifts or drifts. Frequency drift (power is measured within incorrect frequency bins) in optical measurement devices can be due to random or systematic processes (e.g., device aging, thermal fluctuation, etc.) which impair device performance necessary for reliable power measurement used for monitoring and controlling systems. These problems are exacerbated in flex grid systems since it is possible that signals are so close together that they are indistinguishable from the measurement device's perspective. With frequency drift, the wrong power and spectrum info can be fed back to the several layers of local and sectional optical controllers to act ultimately in the wrong direction.
Conventionally, there are several techniques available for correcting the frequency offsets on the optical measurement devices. Most OCMs use an out-of-band internal laser with known frequency and re-calibrate the device from time to time. However, a single out-of-band laser may not be good enough to correct for the frequency offset tilt that can appear across the spectrum. Additionally, the laser, that is used to re-calibrate the device, also suffers from aging effect and may require re-calibration over time, which then has to be done in an out-of-service fashion.
In flexible grid systems, optical channels can be closely spaced with each other in Nyquist or super-Nyquist spacing. In terms of techniques for detecting frequency offsets for flexible grid spectrum, it is extremely difficult and erroneous to detect peak frequency or center frequency for an individual optical channel or its respective edges from the measured optical spectrum as typically measured in the field using low-resolution OCMs. No known methods are available at this point that can successfully detect frequency offsets for individual flexible grid optical channels over the spectrum with various signal bandwidths and signal power varying conditions.